We get really heavy rain (obviously since it's FL) and so our sills around our house have taken a beating. I had thought the previous owners had painted everything right before we moved in, but based on the wear, my memory is wrong or they only painted it with one coat of indoor paint. Regardless, every sill was chipped so we scrapped them all down.
After two coats of primer and paint, everything was looking much better. Since the sills really do get a beating, I went with an oil based primer and an exterior paint that boasts superior protection. We'll see how it actually holds up.